1. Field of the Invention
Embodiments of the present invention relate to a fixing device that fixes an image to a recording medium and an image forming apparatus, such as a copier, a printer, a facsimile machine and a multifunctional device having functions of the copier, printer, and facsimile machine, which includes the fixing device.
2. Description of the Related Art
In the image forming processes performed for known electrophotographic image forming apparatuses, an electrostatic latent image is formed on the surface of a photoconductor drum that serves as an image carrier, developed into a visible toner image by applying a developer thereto, transferred onto a recording medium by a transfer device, and fixed to the recording medium by applying pressure and heat in a fixing device.
The fixing device includes two opposed rollers, a roller and a belt, or a rotary body that is a combination of the rollers or the roller and the belt to sandwich the recording medium therebetween and apply heat and pressure for fixing the toner image to the recording medium. For example, a known belt fixing unit as illustrated in FIG. 1 includes a fixing roller 1B provided with a fixing belt 1J and a pressure roller 1A. The fixing belt 1J includes a heat roller 1D that has a heater 1F serving as a heat source, and the fixing belt includes the fixing roller 1B having a surface layer and a rubber layer. The pressure roller 1A is disposed in contact with the fixing belt 1B. After a recording medium 1H having the transferred toner image thereon has arrived at a fixing device 100, the recording medium 1H is conveyed to a fixing nip contact portion formed between the fixing belt 1J and the pressure roller 1A. In the process in which the recording medium 1H passes the fixing nip contact portion, the toner image transferred onto the recording medium 1H is heated and pressed.
For another example, Japanese Patent Application Publication No. H04-44075 (JP-H04-044075-A) discloses a fixing device 200 that can be used for an image forming apparatus and includes a fixed member that slidably contacts an inner surface of the fixing member, as illustrated in FIG. 2. This fixing device 200 includes a film heating system, in which a fixing nip contact portion is generally formed by sandwiching a fixing film made of heat-resistant film between a ceramic heater 2F serving as a heating element and a pressure roller 2A serving as a pressing member. A recording medium having an unfixed toner image is conveyed between the film and the pressure roller 2A at the fixing nip contact portion, and is further conveyed together with the film, so that heat of the ceramic heater 2F is applied at the fixing nip contact portion to the recording medium via the film. Further, the unfixed toner image is fixed to the surface of the recording medium by application of heat and the pressure exerted at the fixing nip contact portion.
The fixing device 200 employing the film heating system can include an on-demand type device using the ceramic heater 2F and a low heat capacity member such as the film, and can power up the ceramic heater 2F as a heat source in image formation of the image forming apparatus to ensure that the fixing device 200 is heated to the predetermined fixing temperature. Accordingly, a waiting period from power-on to standby for image formation is short (i.e., quick start) and power consumption during standby is greatly reduced.
For yet another example, Japanese Patent Application Publication No. H08-262903 (JP-H08-262903-A) discloses a fixing device employing a pressure belt. The fixing device includes a rotary heating/fixing roll whose surface is elastically deformed, an endless belt that is movable while it is contacting the heating/fixing roll, and a pressure pad that is non-rotatably disposed inside the endless belt to cause the endless belt to press against the heat fixing roller to form a belt nip contact portion between the endless belt and the heat fixing roller so that the recording medium can pass through the belt nip contact portion and can elastically deform the surface of the heat fixing roller. The fixing device using the pressure belt can increase the contact area of the heating/fixing roll with the recording medium, and therefore thermal conductivity of the fixing device can be enhanced and energy consumption can be reduced, thereby reducing the size of the fixing device and the image forming apparatus.
However, the fixing device disclosed in JP-H04-044075-A has insufficient wear resistance for sliding or moving between the ceramic heater 2F as the heat source and the inner surface of the endless belt 2J. For example, if the fixing device 200 is operated for an extended period of time, the inner surface of the endless belt 2J suffers wear due to the friction of contact of the ceramic heater 2F with the endless belt 2J, thereby serving to increase frictional resistance. Accordingly, the movement of the endless belt 2J becomes unstable or the drive torque of the fixing device 200 increases, adversely affecting conveyance of the recording media and causing image displacement. Further, stress on the driving gear can damage the gear train.
Further, the fixing device 200 employing the film heating system is configured to heat the belt at the nip contact portion locally. Therefore, when the belt rotates through the nip contact portion, the temperature of the endless belt 2J is at its lowest and the endless belt 2J has completely cooled down at the entrance to the nip contact portion, and therefore risk of fixing failure increases considerably. This effect becomes more pronounced when the endless belt 2J is rotated at higher speed.
Meanwhile, to improve slidability of the inner surface of the belt and the fixed member, JP-H08-262903-A discloses a configuration of the fixing device in which an outer circumferential surface of the pressure pad is formed as a layer of a fiberglass sheet impregnating polytetrafluoroethylene (a PTFE-impregnated glass cloth) as a low-friction sheet. JP-H08-262903-A describes that the slidability can be enhanced with this method. However, in the fixing device employing the above-described pressure belt, the fixing roller has a large heat capacity, and therefore the temperature of the fixing device does not elevate quickly, which results in a long warm-up time.
To address the above-described problems, the inventors of the present invention have studied the fixing device that is disclosed in Japanese Patent Application Publication No. 2007-334205 (JP-2007-334205-A). This fixing device discloses technology in which the entire part of a sleeve endless belt is heated by a heat source disposed inside the belt, thereby shortening a warm-up time of the fixing device. However, the sleeve endless belt includes the heat source therein and therefore does not have a drive shaft therein. With this configuration, the entire fixing unit needs to be replaced instead of just the endless belt, resulting in an increase in costs.